In the presently known methods of manufacturing plastic materials, it is a well-known fact that the mechanical properties of polymers change substantially when the molecular chains (threadlike molecules) from which they are composed of, are not left in their natural chaotic (convoluted) arrangement, but are orientated. Generally, the orientation results in enhancement in strength of the articles in a given direction by orienting the molecules in the plastic material in respective direction. Usually, biaxial orientation of the plastic materials is preferred in order to improve the mechanical properties through molecular orientation of the thermoplastic material in two mutually perpendicular directions—the axial direction and the circumferential or radial direction.
In case of plastic pipes, the molecular orientation is conferred in the circumferential direction (radial orientation) in order to increase the pressure resistance of the plastic pipe; and in the longitudinal direction (axial orientation) in order to increase the tensile strength of the pipe. More specifically, the orientation in the radial direction provides a high admissible stress at yield (i.e. high elastic limit). The higher the admissible stress at yield, the lower the thickness of the tube wall required to resist a given internal pressure. Similarly, when the molecular orientation is in axial direction, the stress at yield is much higher in axial direction but also the admissible elongation before break (strain) is much lower. However, due to possible incidents during manipulation of the irrigation tube, the elongation before break (strain) must be as high as possible.
Whereas no specific requirement has been realized for the high stress at yield in axial direction of the tube. Therefore, it is highly desired that the tubes are oriented absolutely in radial direction with minimal or no axial orientation in order to produce a tube with high stress at yield in radial direction (to withstand high internal pressure) and high elongation before break (to withstand longitudinal stress during the tube manufacture). Such tubes have high utility in the drip irrigation systems. However, with the presently available set of machines used for the production of irrigation tubes, it is not possible to achieve the desired composition of molecular orientation and/or thickness of tube wall in the final manufactured tubes while still maintaining the desired high speed of production.
Among several known methods and apparatuses available in the prior art for manufacturing of oriented plastic materials, the blown-film technique is most commonly used for manufacturing plastic films having a combinations of radial and axial orientation. In the blown-film technique, the molten polymer from the extruder head enters the die, where it flows round a mandrel and emerges through a ring-shaped opening in the form of a tube. The tube is expanded into a bubble of required diameter by the pressure of internal air admitted through the center of the mandrel. The air contained in the bubble acts like a permanent shaping mandrel.
This way, the molecular orientation is obtained in the film in hoop direction (radial orientation) during blowup; and additional orientation in the direction of flow (longitudinal orientation) can be induced by tension from the rollers located downstream. The film bubble moves forward through guiding devices into a set of pinch rolls which flatten it. However, the blown film technique is generally employed for manufacturing the tapes and films; it is not practiced for manufacturing long tubes or pipes.
The methods and apparatuses presently used in industry for manufacturing irrigation tubes are based on pipe/tube extrusion methods and/or variations thereof. In these methods, the die used for the extrusion of pipe or tubing comprises a die body with a tapered mandrel and an outer die ring which control the dimensions of the inner and outer diameters of the tube, respectively. Typically this die plate (having an orifice of appropriate geometry), placed on the face of the die assembly is called the ‘Calibrator’ which constitutes an indispensable part of the presently used tube-extrusion machines. While passing through the calibrator, the molten polymer is subjected to high surface drag, resulting in high friction and reduced flow through the thinner sections of the orifice. Thus the calibrator in the production line, although essential for determining the tube parameters, acts as a limitation factor for the production speed. The surface drag results in a reduced rate of production of the irrigation tube, especially in case of production of tubes with low diameter. Although this effect can be countered by altering the shape of the orifice, but this often results in a wide difference in the orifice shape from the desired extrusion profile. Therefore, this problem of reduced flow is presently countered in the industry by the use of vacuum suctions immediately downstream of the calibrator. This vacuum suction, although increases the speed of passage of the extruded tape, further increases the friction and thus the extruded tube is subjected to high longitudinal stretch and the axial expansion. This inadvertently results in undesired excessive axial orientation of the molecular chains in the final produced tubes. Thus the calibrator, although an extremely essential component for the presently used machines, acts as an impediment to the speed of production and any effort made thereby to increase the speed of production would tend to compromise the mechanical properties of the tube by exerting unnecessary longitudinal stretching (axial orientation) to the formed tube. Although an already oriented tube can be further subjected to a stabilizing process which includes reheating and inflating the tube to a pre-established diameter and subsequently cooling it in a cooling chamber containing a calibrating device capable of determining the finished diameter of the tube, but that would be a much lengthy process and certainly not cost effective. Therefore, the presently used tube-extrusion methods do not take into account the excessive axial orientation occurred during the production process and are thus not appropriate for conferring desired molecular orientation at high speed production and low cost.
Thus, in the presently used systems the speed of production of drip irrigation tubes is to be compensated against the requirement of minimal axial stretch. It is further difficult to manufacture the small-diameter tubes at high speeds and at desired orientation of molecular chains, because as the tube diameter decreases the surface drag increases and the rate of flow or passage decreases accordingly. In such case, the longitudinal stretch becomes unavoidable which further increases with the use of vacuum suction for increasing the speed of production.
Further, since the degree of radial orientation determines the tube stiffness to withstand the internal fluid pressure, more the axial orientation of tube, lesser will be the radial orientation and more will be the wall thickness of tube which is required to withstand a given internal pressure. Therefore, in light of the above described methods and systems of the prior art, where a considerable amount of longitudinal stretch (and thereby caused axial orientation) is unavoidable, unless partially compensated with the speed of production, more wall thickness is required to withstand the given internal pressure of fluid due to reduced radial orientation. This increased wall thickness results in extra consumption of raw material and thereby extra cost of production.
Even further, this also results in the non-uniformity and complexity in the production process.
In addition to the above, due to more thickness of the tube wall in the tubes produced in the prior art systems, biaxial stretching is required to be carried out at a considerable distance downstream of the calibrator. Therefore, it is required to have plurality of heating and cooling panels in succession to reheat the tube in order to obtain the desired stretching and/or thickness of the tube. Even further, usually the plastic tubing must be rotated in order to obtain some degree of uniform heating. Due to the foregoing provided drawbacks in the prior art, the amount of time necessary to get the plastic tubing to the required temperature is significantly high.
It is also known in the prior art to fabricate plastic tubes from a pliable plastic strip folded lengthwise. Two edges of the tape are overlapped and joined together to form a flat hollow tube. Subsequently, under pressure the tape opens out into a generally cylindrical form to provide a main conduit. Usually, the drip irrigation tape also includes a much smaller secondary conduit located along the seam formed by the overlapping edges of the plastic strip. The smaller conduit is connected to the main conduit to form a narrower passageway for the water flow, reducing the rate of flow of liquid. However, the method for making the films and sheets suffer from the abovementioned drawbacks.
There are methods for manufacturing drip irrigation tubes from films of with laminated bubble approach; for example, U.S. Pat. No. 5,591,293. Further, there are methods of manufacturing irrigation hoses from polymer beads using conventional extrusion nozzle; for example U.S. Pat. No. 4,642,152 (the Chapin method). However, these methods have certain limitations. The Chapin method, which employs conventional extrusion nozzle, is expensive and less efficient. Further, if the Chapin method is to be used for manufacturing tubes made up with two materials (one forming a main body and the other forming a pressure responsive/pressure-compensator membrane), then there are certain limitations. For instance, positioning the pressure-responsive membrane in the appropriate position is technically very difficult. If the positioning is not correct then the Chapin process does not render efficient and accurate results. Another drawback of the method is that it involves bonding of the flow path with the teeth; however, pressure compensation obtained by this method is comparatively less responsive. Further, the methods known in the prior art do not address the requirement of obtaining strength without adding stiffness. Thus, there is need for a process that allows fast production of drip irrigation tubes made up of two different materials, particularly including such tubes that are thin-walled and are with smaller diameters.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for the high-speed production of drip irrigation tubes while still enabling to achieve the desired minimal or zero longitudinal stretch of tube, and thereby caused minimal or zero axial orientation of molecular chains. Particularly, it is the object of the present invention to provide a method and an apparatus for high speed production of thin-walled pressure-compensator-type drip irrigation tubes with smaller diameters having desired radian and axial orientation of its molecular chains.
It is an object of the present invention to present a method that allows optimum utilization of the Chapin process for producing pressure-compensator type drip irrigation tubes made from two different materials (one forming a main body and the other forming a pressure responsive/pressure-compensator membrane) by allowing correct positioning of the pressure-responsive membrane with respect to the main body of the tube. The correctness of the positioning is ensured by co-extruding the pressure-responsive membrane together and coincidental with the main body material and thereafter folding the membrane at an appropriate place.
Further objects and advantageous embodiments of the present invention are disclosed in the appended claims and the following description.